This invention relates to an image forming apparatus and control device and method therefor, and in particular to an image forming apparatus capable of reducing irregularities in the printed images caused by the fluctuations in the speed with which sheets of paper are transported at the time of printing, as well as control device and method for such a printer. The term xe2x80x9cimage forming apparatusxe2x80x9d as used herein will include all kinds of apparatus adapted to print an image on a paper sheet or transfer an image onto a paper sheet. In other words, ordinary printers, copiers and facsimile machines are included within the meaning of the expression.
With the recent progress in technology, it is becoming possible to make many prints at a fast rate with a printer of a relatively small size. If a printer is made smaller, however, the distance of travel by the paper sheet becomes shorter, or the length of the path between the place of paper supply and the place where the printing takes place. Thus, the printing device becomes more susceptive to the effects of transportation load in the paper supply part and the travel path of the paper sheets. Accordingly, it becomes difficult to transport a paper sheet smoothly and the speed of paper transport begins to fluctuate. This has been causing phenomena such as distortions and graduation in the image printed on such a paper sheet.
It is therefore an object of this invention to reduce the irregularities in printed images caused by the fluctuations in the speed of transport of paper sheets.
An image forming apparatus embodying this invention, which incorporates a control device of this invention for controlling its operations by a method of this invention and with which the above and other objects can be accomplished, may be characterized in most basic terms as comprising rotary devices such as a drum and a roller pressed against it for printing on a paper sheet while rotating and transporting it, a paper-transporting mechanism for transporting paper sheets one by one to the rotary devices and a control device for controlling the operation of the paper-transporting mechanism. According to a first embodiment of the invention, the control device serves to control the paper-transporting mechanism so as to transport the paper sheet at a faster speed than a slower speed by the rotary devices. As a result, there is no braking effect on the paper sheet from the paper-transmitting mechanism as it is being printed on and transported by the rotary devices and irregularities in the printed image can be prevented. The paper-transporting mechanism may include guide rollers, a motor for rotating them and a clutch for switching on and off the transmission of rotation of the motor to the guide rollers. The control device controls the operation by starting to rotate the motor before the front edge of a paper sheet reaches the rotary means and switching on the clutch when the front edge of the paper sheet does reach the rotary means. By a control of this type, the motor can be started preliminarily and hence the paper sheets can be quickly transported at a specified speed.
The paper-transporting mechanism may be described as comprising a first mechanism for transporting the paper sheet to the rotary devices and a second mechanism for transporting the paper sheet to the first mechanism, say, from a tray for preliminarily storing paper sheets. The first mechanism may comprise a pair of what is commonly referred to as the timing rollers. The second mechanism may comprise a pair of guide rollers to be driven by a motor through a clutch, as explained above regarding the first embodiment. According to a first embodiment of this invention, the second mechanism is controlled so as to transport the paper sheet not only at a faster speed than the speed of transportation by the rotary devices but also while the first mechanism is released from the transportation of the paper sheet. According to a second embodiment of the invention, the control device serves to release the second mechanism from the transportation of the paper sheet when the paper sheet comes to be in a transportable condition by the rotary devices. Instead of having a pair of guide rollers both of which are adapted to be driven by a motor, the second paper-transporting mechanism may have a driver roller driven by a motor and a follower roller which is normally pressed against the driver roller by the biasing force of a spring and a power source such as a solenoid adapted to remove the follower roller away from the driver roller, when activated. The timing of operation may be variably set according to the characteristic of the paper sheet such as its size and paper quality (such as its weight per unit surface area) such that irregularities in the printed image can be reliably prevented, independent of the type of the paper sheets being used.